support local output in build

Signed-off-by: Evan Hazlett <ejhazlett@gmail.com>

README.md

@@ -33,6 +33,11 @@ To build an image using BuildKit:
 # nerdctl run -it --rm foo
 ```
 
+To build and send output to a local directory using BuildKit:
+```console
+# nerdctl build -o type=local,dest=. /some-dockerfile-directory
+```
+
 To run containers from `docker-compose.yaml`:
 ```console
 # nerdctl compose -f ./examples/compose-wordpress/docker-compose.yaml up
@@ -119,7 +124,7 @@ Major:
 - [Running encrypted images using ocicrypt (imgcrypt)](./docs/ocicrypt.md)
 
 Minor:
-- Namespacing: `nerdctl --namespace=<NS> ps` . 
+- Namespacing: `nerdctl --namespace=<NS> ps` .
   (NOTE: All Kubernetes containers are in the `k8s.io` containerd namespace regardless to Kubernetes namespaces)
 - Exporting Docker/OCI dual-format archives: `nerdctl save` .
 - Importing OCI archives as well as Docker archives: `nerdctl load` .
@@ -261,7 +266,7 @@ It does not necessarily mean that the corresponding features are missing in cont
 <!-- END doctoc generated TOC please keep comment here to allow auto update -->
 
 
- 
+
 ## Run & Exec
 ### :whale: nerdctl run
 Run a command in a new container.
@@ -505,7 +510,7 @@ Flags:
 Unimplemented `docker inspect` flags:  `--size`, `--type`
 
 ### :whale: nerdctl logs
-Fetch the logs of a container. 
+Fetch the logs of a container.
 
 :warning: Currently, only containers created with `nerdctl run -d` are supported.
 
@@ -718,7 +723,7 @@ Create a network
 Usage: `nerdctl network create [OPTIONS] NETWORK`
 
 Flags:
-- :whale: `--subnet`: Subnet in CIDR format that represents a network segment, e.g. "10.5.0.0/16" 
+- :whale: `--subnet`: Subnet in CIDR format that represents a network segment, e.g. "10.5.0.0/16"
 - :whale: `--label`: Set metadata on a network
 
 Unimplemented `docker network create` flags: `--attachable`, `--aux-address`, `--config-from`, `--config-only`, `--driver`, `--gateway`, `--ingress`, `--internal`, `--ip-range`, `--ipam-driver`, `--ipam-opt`, `--ipv6`, `--opt`, `--scope`

cmd/nerdctl/build.go

@@ -17,6 +17,7 @@
 package main
 
 import (
+	"fmt"
 	"os"
 	"os/exec"
 	"strings"
@@ -65,6 +66,11 @@ var buildCommand = &cli.Command{
 			Name:  "no-cache",
 			Usage: "Do not use cache when building the image",
 		},
+		&cli.StringFlag{
+			Name:    "output",
+			Aliases: []string{"o"},
+			Value:   "type=docker",
+		},
 		&cli.StringFlag{
 			Name:  "progress",
 			Usage: "Set type of progress output (auto, plain, tty). Use plain to show container output",
@@ -106,8 +112,10 @@ func buildAction(clicontext *cli.Context) error {
 		return err
 	}
 
-	if err = loadImage(buildctlStdout, clicontext); err != nil {
-		return err
+	if !isLocalBuild(clicontext) {
+		if err = loadImage(buildctlStdout, clicontext); err != nil {
+			return err
+		}
 	}
 
 	if err = buildctlCmd.Wait(); err != nil {
@@ -131,7 +139,7 @@ func generateBuildctlArgs(clicontext *cli.Context) (string, []string, error) {
 		return "", nil, err
 	}
 
-	output := "--output=type=docker"
+	output := fmt.Sprintf("--output=%s", clicontext.String("output"))
 	if tagSlice := strutil.DedupeStrSlice(clicontext.StringSlice("tag")); len(tagSlice) > 0 {
 		if len(tagSlice) > 1 {
 			return "", nil, errors.Errorf("specifying multiple -t is not supported yet")
@@ -180,3 +188,7 @@ func generateBuildctlArgs(clicontext *cli.Context) (string, []string, error) {
 
 	return buildctlBinary, buildctlArgs, nil
 }
+
+func isLocalBuild(clicontext *cli.Context) (bool, error) {
+	return len(strutil.DedupeStrSlice(clicontext.StringSlice("tag"))) > 0
+}

cmd/nerdctl/build_test.go

@@ -45,6 +45,32 @@ CMD ["echo", "nerdctl-build-test-string"]
 	base.Cmd("run", "--rm", imageName).AssertOutContains("nerdctl-build-test-string")
 }
 
+func TestBuildLocal(t *testing.T) {
+	testutil.RequiresBuild(t)
+	base := testutil.NewBase(t)
+	const testFileName = "nerdctl-build-test"
+	outputDir, err := os.MkdirTemp("", "nerdctl-build-test-")
+	assert.NilError(t, err)
+	defer os.RemoveAll(outputDir)
+
+	dockerfile := fmt.Sprintf(`FROM scratch
+COPY %s /`,
+		testFileName)
+
+	buildCtx, err := createBuildContext(dockerfile)
+	assert.NilError(t, err)
+	defer os.RemoveAll(buildCtx)
+
+	if err := ioutil.WriteFile(filepath.Join(buildCtx, testFileName), []byte("nerdctl-test"), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	base.Cmd("build", "-o", fmt.Sprintf("type=local,dest=%s", outputDir), buildCtx).AssertOK()
+	if _, err := os.Stat(filepath.Join(outputDir, testFileName)); err != nil {
+		t.Fatal(err)
+	}
+}
+
 func createBuildContext(dockerfile string) (string, error) {
 	tmpDir, err := ioutil.TempDir("", "nerdctl-build-test")
 	if err != nil {